1. Field of the Invention
This invention relates to a coordinate reader (generally referred to as a digitizer) based on electromagnetic coupling for detecting coordinates of a cursor.
2. Description of the Prior Art
Conventionally, it has been practiced to detect coordinates of a cursor by A-D converting the amplitude of a voltage induced in a magnetic flux detecting coil or in conductor loops formed on an electrically insulating board, or by measuring an interval of time until occurrence of phase inversion of an induced voltage. However, the former method faces a problem that the magnitude of the induced voltage changes with a slight deflection of the cursor from the electrically insulating board and a means for correcting the height of the cursor is needed. The latter method can be dispensed with the cursor height correcting means as required for the former method but suffers from a problem that accuracies of the coordinate detection based on the time measurement are directly affected by temperature dependent changes in constants of circuit parts. For these reasons, both the methods encounter difficulties in highly accurate coordinate reading and besides suffer expensiveness.